1. Field of the Invention
The present invention relates to a toner for color electrophotography and a fixing method using the above toner. More specifically, the present invention relates to a toner for color electrophotography which is simultaneously fixed with different color toners and has viscoelastic properties approximated to a level of the other toners, and a fixing method using the above toner and a black toner.
2. Discussion of the Related Art
Toners may be classified into black toners used in normal monochromatic processes, and color toners used in full color processes and two-color printing processes. An example of monochromatic processes include Carlson method, comprising the steps of evenly charging a photoconductive insulating layer (a charging process); subsequently exposing the layer to eliminate the charge on the exposed portion, to thereby form an electrostatic latent image (an exposing process); visualizing the formed image by adhering colored charged fine powder, known as a toner, to the latent image (a developing process); transferring the obtained visible image to an image-receiving sheet such as a transfer paper (a transfer process); and permanently fixing the transferred image by heating, pressure application or other appropriate means of fixing (a fixing process). In cases of two-color printing processes, two different color toners have to be used for repeating the above steps, and the toners have to be fixed simultaneously. Also, in full color processes, as in the case of the two-color printing processes, a number of color toners are subjected to similar steps and then allowed to be fixed simultaneously.
In the above fixing process, since the toners are demanded to have excellent fixing ability and offset resistance, a desired fixing ability is secured by controlling viscoelastic properties generally by selecting a kind of binder resins or adding various kinds of additives. For instance, in the case of black toner, carbon black is conventionally added to obtain a good reinforcing effect so that a given level of viscoelastic properties is obtained, to thereby secure the desired fixing properties in the resulting toner. Also, fixing apparatuses are designed to match the fixing properties owned by the toners.
However, in cases of color toners, since colors are greatly changed by the addition of carbon black, the carbon black cannot be similarly added to color toner as in the case of the black toner, thereby making it impossible to reinforce the color toners using carbon black. Also, coloring agents used for the color toners cannot exhibit good reinforcing effects as in the case of adding carbon black so that mechanical properties such as elastic modulus are somewhat lowered in the color toners by an extent that the carbon black can reinforce them. Thus, at present, since thermal properties of the binder resins usable for color toners and fixing mechanisms utilized therefor are quite limited, it has been known to be quite difficult to fix both kinds of the toners at the same time, while providing the fixing ability of the color toners approximated to a level of the fixing ability of the black toner.
On the other hand, it is well known that when the amount of carbon black to be blended in black toners is controlled so as to change the reinforcing effects, the changes in triboelectric charge, resistivity, and toning are undesirably likely to take place. Therefore, it is generally difficult to adjust the amount of carbon black in the black toner so as to approximate the values of viscoelastic properties of the color toners to those of the black toner.
Accordingly, in the field of art, a reinforcing agent for toners which does not change coloring and toning of the toners when used for color toners and is capable of controlling the viscoelastic properties of the toners is in strong demand.
Various proposals have been made to improve the toner properties by incorporating inorganic fine particles, such as silica fine particles, in the toner. For instance, Japanese Patent Laid-Open No. 54-20344 discloses a negatively chargeable powdery toner for electrophotography containing hydrophobic silica fine particles, wherein the amount of the hydrophobic silica fine particles is preferably 10 to 40% by weight. However, in this reference, the hydrophobic silica fine particles are added for the purpose of controlling the chargeability of the toners to negative polarity, not for the purpose of controlling the viscoelastic properties of the toners.
In addition, in other art disclosing the addition of silica fine particles in the toners, the addition is made for the purposes of controlling negative chargeability and of improving blocking resistance and fluidity (see Japanese Patent Laid-Open Nos. 51-81623, 52-30437, and 58-27503). Therefore, once again, the additions are not made for the purpose of controlling the viscoelastic properties of the toners.